Single cylinder deadbolt lock mechanisms having a thumbturn at the side opposite the cylinder lock are well known. Conventional locks of this type provide for extending and retracting the latch bolt by use of either the lock cylinder or the thumbturn. A recognized disadvantage of such lock mechanisms is that a burglar or other such unauthorized person who gains entry to a building may easily exit the same through the door after simply unlocking the same from the inside by use of the thumbturn. For doors with windows, entry to the building is readily gained after first breaking the window and unlocking the door from the inside thereof. Although double cylinder deadbolt locks often are used to prevent such easy exit from a building, they are more expensive than single cylinder locks and, in case of fire or other emergency, may prevent quick egress from the building.
Additionally, single cylinder deadbolt lock mechanisms are known wherein the thumbturn is rendered inoperative when the latch bolt is moved into locking position by key operation of the lock cylinder. Such a lock mechanism is shown in U.S. Pat. No. 2,178,666 issued to O.C. Larson. As with the above-mentioned prior art single cylinder locks of this general type, a major disadvantage of such an arrangement is that the door can not be opened from the inside using the thumbturn after being locked by use of the key from the outside. Any authorized person in the building could not exit through the door once the door is locked by operation of the lock cylinder. An arrangement which avoids this disadvantage is shown in U.S. Pat. No. 4,047,408 issued to Eddie D. Johns, et al wherein the thumbturn is selectively rendered inoperable when the cylinder lock is operated by the key. With this arrangement, if the key is depressed while locking the door, the thumbturn is rendered inoperative and, if not depressed, the thumbturn remains operative. Major disadvantages of such an arrangement include the complexity and cost of manufacturing the same which greatly limit the marketability thereof. Also, the thumbturn disabling mechanism is operated from the cylinder side of the lock rather than from the thumbturn side, thereby contributing to the complexity thereof.